Plasmonics with a Twist: from Single Particles to Metasurfaces
Nanophotonics deals with how electromagnetic fields can be confined and squeezed to orders of dimensions below the light wavelength. Plasmonics, one of the fields of nanophotonics, study collective free electron oscillations driven by light in metallic nanostructures and interfaces. The greater part of plasmonics applications utilizes the plasmons light localization property and the resulting enhanced photon density of states. Plasmonic effects can also be used to harvest and modify photon angular momentum. In this thesis, plasmonics is used in two different ways: 1) to induce rotation in optically trapped gold nanoparticles, and 2) to create chiroptically active metasurfaces.
Optical tweezers was pioneered in the early 1970s by the work of A. Ashkin. As its name suggests, optical tweezers allow for precise control of the position and movement of small objects with light. This can be used for manipulation of fragile samples, such as living cells, and nanoparticles with nanometer-level precision, as well as for sensing of forces in the piconewton range. Adding rotation frequency control to the set of features applicable to a trapped particle could expand the utility of optical tweezers to nanoscale viscosity probing, mixing in micro- and nanofluidics, and microtooling of soft matter. In this thesis, gold nanoparticles, trapped by a 2D optical trapping setup against an interface, were set in rotation using a circularly polarized laser beam. The absorbed and scattered photons possess an intrinsic spin angular momentum that generate an optical torque, which is balanced by a drag force from the surrounding water. The power dependence of the rotation frequencies could be well fitted by classic electromagnetism and hydrodynamic theories. Additionally, the light induced heating of the nanoparticles at higher laser powers was shown to strongly influence the friction of the enclosing liquid. New measurements on nanorods rotating against a water-oil interface showed increased rotation frequencies, possibly due to a particle orientation change. Rotating nanoparticles could offer a new route to probe interfaces as well as being nanoscale sources of emulsion.
In order to manipulate photon angular momentum, nanoparticle metasurfaces were fabricated with a focus on breaking their overall geometrical symmetry. The main technique used was hole-mask colloidal lithography, which is low cost, high speed, and offers large area coverage. Based on this method, and thanks to metallic angular and rotating evaporation techniques, diverse nanostructure patterns with increased feature control were developed. Starting from a simple geometrical principle, chirality, two types of metasurfaces were investigated. Both showed pronounced optical activity, that is, different optical response to illumination with right- or left-handed circularly polarized light. In the case of tetramers built from particles with different height, forming a staircase, an intrinsic chiral response was induced through strong near-field coupling and phase retardation effects. In the case of anisotropic particle layers, an evanescent illumination configuration resulted in extrinsic chirality, which allowed for a contrast of around 90% in reflection between right- and left-handed circularly polarized light. This effect arises from the illumination geometry, the polarization-dependent excitation efficiency of the nanoparticles with respect to their orientation, and the density of dipoles on the surface. The observed phenomena might be utilized in, for example, polarization manipulation and enantiomer sensing.
Taken together, the results presented in this thesis offer some new insights into the fascinating interaction between circularly polarized light and plasmonics nanoparticles. The results may also provide as well a platform for further development of complex nanostructures for basic studies and diverse plasmonic applications.
localized surface plasmons
circularly polarized light